Vegetation in the westernmost third of the Sonoran - the lowest and driest part of the desert - is mostly creosote bush and bursage with widely scattered mesquite, palo verde, ironwood, and catclaw. A variety of cacti punctuate this landscape.

The remaining two - thirds of the Sonoran Desert, to the east, lie at higher elevations and support a much greater density of succulent plants. Here, the giant saguaro flourishes, growing in forestlike stands interspersed with thickets of mesquite, acacias, ironwood, and palo verde. The bursage and creosote growing at lower elevations give way to yuccas, agaves, beargrass, barrel cactus, cholla, and prickly pear. Oak, juniper, and pinyon pine grow on the mountains rising above the desert floor. The seeds, stems, and leaves of many of these plants provide rich food sources, sustaining populations of deer, bighorn sheep, javelina, reptiles, rabbits, and other rodents.

The saguaro, the largest cactus found in the United States, is the symbol of the Sonoran Desert, even though it does not occur throughout this desert. The largest of the saguaros can tower 15 meters above the desert floor and weigh 6 to 7 metric tons at 150 to 200 years of age.Saguaros take an incredibly long time to mature. After 10 years, they may measure only 10 to 15 centimeters. The first flowers don't appear until the plant is 30 to 40 years of age. The distinctive armlike branches may not appear until the saguaro is almost 75 years of age.The saguaro can absorb 760 liters - an amount that will allow the cactus to survive for a year - from a single, infrequent rainstorm. What allows the saguaro to absorb water so quickly is its extensive but shallow root system, which may extend outward from 9 to 18 meters in all directions. This system allows the saguaro to thrive, but also permits the plant to be easily uprooted by cactus thieves, who can earn more than a $1,000 from the sale of a single desert giant. Saguaros are an important focal point for many animal species. Most saguaros are pockmarked with round holes drilled by Gila woodpeckers and northern flickers searching for larvae or hollowing out nest sites. Abandoned holes become homes for screech owls, pygmy owls, or tiny elf owls.

Photo by Diane Drobka

Perhaps the most recognizable symbol of the Sonoran Desert is the giant saguaro cactus, but many other cacti add their distinctive shapes and spectacular flowers to this landscape, including the tall columns of the organ pipe and senita and the squat forms of the cholla and prickly pear. The ocotillo's thorny, sticklike branches rise from the ground, ready to sprout green leaves at the first touch of rain. Burro brush, brittlebush, desert broom, and sotol fill in beneath taller plants. Agave - once cultivated as an important food crop by the Hohokam Indians a thousand years ago - also grows here.

Ringtail cats are related to the raccoon and are strictly nocturnal animals. About the size of a housecat, miners often kept them to catch mice.

Photo by Marty Cordano

What makes the Sonoran Desert such a place of abundance? In a word, water. The Sonoran Desert has two rainy seasons: winter and summer. The winter rains of December through March come from the west, while the summer rains move east from the Gulf of Mexico, primarily in July and August. These summer "monsoon" rains bring awesome thunder and lightning storms accompanied by drenching downpours that briefly break the intense heat and cause the desert to bloom anew.In addition to having two rainy seasons, this arid region is bordered by mountains in Colorado, Arizona, and the Mexican state of Sonora, which receive much greater rainfall and snowfall. The water trapped by these mountains drains into rivers that cross the desert, some of them maintaining corridors of riparian vegetation during even the driest months of the year. The Colorado River runs year-round, and the Gila River was a nearly constant stream before dams were built on it. Until the mid-nineteenth century, the San Pedro River ran year-round, and the Santa Cruz River ran intermittently with long stretches of surface flow. Sonoran plants and animals have developed some unique and innovative ways to survive. The roots of the mesquite tree bore as deep as 30 meters to find moisture. Some hardy desert perennials simply die back above ground when the moisture regime is unfavorable. Such plants survive through a variety of remarkable underground structures, such as thickened roots, bulbs, tubers, rhizomes, and nodules of many designs and dimensions.

The crested caracara, also known as the Mexican eagle, is often seen majestically perched on saguaro or organ pipe cacti. It typically does not hunt for food, eating carrion instead.Photo by BLM

The ironwood tree produces leaves during favorable moisture conditions, but rapidly sheds them under moisture stress, opting instead to conduct photosynthesis in its trunk and branches. The ocotillo, too, has high water needs when it is in full leaf, but, at the first hint of drought, it sheds its leaves. The creosote bush has a highly competitive root system capable of drawing almost all available moisture from the soil. The uniform spacing of creosote bushes, as seen from the air, is the result of root competition for the small amounts of water available in the area.Some old and strong desert shrubs conduct chemical warfare against younger and weaker plants. The leaves of the brittlebush and guayale poison the seedlings of many other plants, prohibiting their growth. Seeds of the palo verde, ironwood, and smoke trees have extremely hard coats. Before these seeds can germinate they must be abraded; this occurs when the seeds lying in the bed of a wash (a dry river bed) are caught up in the occasional flood. If sufficient water is still present, the plant will grow. Other plants, called drought evaders, don't resist drought but avoid it altogether. These plants persist only as seeds, ready to spring up when it rains, flower quickly, produce another crop of seeds, and then die.Animals, too, have developed special tools for survival. Among the best adapted is one of the smallest of all desert mammals, the kangaroo rat. These rodents can make long, swift leaps with their powerful hind legs to escape predators. Using their tails as rudders, they can change course in midair. These mammals can survive their entire lives without drinking a drop of water. Instead, they obtain moisture primarily from dry seeds, which contain up to four percent water, and from an occasional leaf or insect. A complex moisture-conservation system also protects the kangaroo rat from dehydration. Its metabolic system is highly efficient at recycling and retaining water so that very little moisture is lost through urination; what does pass out looks like a concentrated paste or crystals. A specially designed respiratory system cools the rat's breath before it can be exhaled through the nasal passages and condenses the moisture into minuscule droplets, which are then reabsorbed into the rat's body.

Hedgehog cacti flowers. The buds form under the surface of the cactus' ribs and then burst through the tissue.Photo by BLM

One of the best known of desert birds is the greater roadrunner. This bird spends almost its entire life on the ground running, attaining speeds of up to 40 kilometers per hour. Oddly, running uses fewer muscles than flying, making it a less strenuous method of movement. The spadefoot toad is one of the most persistent of the desert dwellers. A horny projection on each hind foot serves as a digging tool. When drought sets in, the toad burrows backward deep into the ground and begins a period of estivation, a state akin to hibernation. Its underground cell is lined with a gelatinous substance, secreted by the toad itself, that greatly reduces water loss. The period of estivation lasts eight or nine months of the year, until a cloudburst saturates the soil, awakens the toad, and creates a frenzy of activity that results in mating and the hatching of tadpoles in a day or two.

Management ChallengesThe land ownership pattern in the Sonoran Desert is a checkerboard of public, tribal, and private ownership. The Bureau of Land Management of the U.S. Department of the Interior oversees large tracts of land in the Sonoran Desert along with other entities, including Indian tribes. Some 30 million people surrounding the desert have a growing interest in these open spaces. More and more, urban and suburban dwellers in Tucson, Phoenix, and Southern California are looking to these public lands for natural resources, recreation, urban development and other uses. The challenge is to manage these varied uses while sustaining the desert's fragile balance.

RecreationSeeking escape from crowded urban areas, many people turn to the Sonoran's open spaces for solace and recreation. Designating special areas for recreation is one strategy land managers are pursuing to cope with the influx of visitors. This strategy was adopted at the Imperial Sand Dunes in Southern California. Thought to be beach sands from ancient Lake Cahuilla, the Imperial Sand Dunes are the most popular destination in the Sonoran. Over a half million visitors come to this "play area" each year to explore the dunes in off-highway vehicles, which are designed for use on unimproved roads or back country areas. In 1994, Congress designated the northern part of the dunes as a wilderness area, while allowing vehicular use in the southern portion. A management plan for the dunes area outlines over 200 actions for managing the visitors and protecting dune resources. Volunteers work with federal and state land managers to patrol the area and conduct education programs promoting public safety and resource conservation. Volunteers also donate over 7,500 hours annually to assist law enforcement officers in protecting the area.

Long-Term Visitor Areas (LTVAs) provide facilities for campers while concentrating them away from the back country where camping would have adverse impacts.

Photo by BLM

The Long-Term Visitor Area (LTVA) program is another effort to concentrate visitors in designated areas, keeping campers from the back country where more impacts would occur. Seeking refuge from winter weather, thousands of people from across the northern United States and the Canadian provinces take up residence on public lands in the Sonoran and Mojave Deserts. In 1983, the Bureau of Land Management established the LTVA program to respond to this annual migration of "snowbirds." For a nominal fee, campers spend the winter months in any one of eight LTVAs. The program has been successful in reducing trash and other disturbances to the desert. At the same time, water, toilets, refuse disposal, and waste dump stations provide a cleaner environment and amenities for visitors.

The Algodones Sand Hills, also known as the Imperial Sand Dunes, are a favorite visitor attraction in Southern California. They are believed to be ancient beach sands.

Photo by RAB

Exotic PlantsSome plants found in the Sonoran Desert today, such as red brome, yellow star thistle, Malta star thistle, buffel grass, and tamarisk (saltcedar), do not belong there. They were introduced from other countries and are called exotic, alien, or non-native species. For example, the tamarisk was planted as a shade tree and to control wind and erosion. It came originally from the Mediterranean area and can tolerate salty soils.In its native range, saltcedar populations are restricted by natural pests, such as insects and diseases. But these pests were not introduced into the United States along with the saltcedar. Thus, weedy saltcedar species can expand their range anywhere the climate is suitable.

A Sonoran landscape with prickly pear and saguaro cactus against a mountain backdrop.

Photo by Diane Drobka

Saltcedar grows well in sandy, moist soils and has invaded riverbanks and natural springs, competing with native species for precious water sources. The two most common native plants displaced by a saltcedar invasion are cottonwoods and willows growing along rivers and streams. In some areas, mesquites are also eventually displaced by saltcedar. Once established, it prevents native grasses, forbs, and shrubs from recovering by exuding salts from its leaves, which increases the salinity of the surrounding soil beyond the tolerance of natives.The invasion of the desert by saltcedar can also affect native wildlife. In the United States, saltcedar is not commonly eaten by native herbivores, such as bighorn sheep and deer, and the plant's seeds are too small to be a food source for birds or rodents. Control and eradication programs are being conducted throughout the western United States. In most cases, the goal is to preserve or recover sensitive areas. Eliminating massive infestations along major rivers is not economically feasible at present.

Threatened and Endangered SpeciesCommunities are also turning to public lands for expansion and development. Protecting unique habitats, particularly for threatened and endangered species, can complicate these plans. The desert slender salamander, peninsular bighorn sheep, desert tortoise, southwestern willow flycatcher (a neotropical bird), and the Nichol's Turk's Head cactus are a few examples of species at risk. In most cases, creative and cooperative efforts provide the solutions. The Coachella Valley Fringe-Toed Lizard Preserve is one such example.

When enemies threaten, the fringe-toed sand lizard dives into the sandy soil and swims out of sight in seconds. Specially constructed scaly fringes on its toes make this vanishing act possible, while a built-in sand trap in its nose enables it to breathe under the sand without choking.

Photo by U.S. Fish & Wildlife Service

Since 1940, the population of the 520 - square - kilometer Coachella Valley in Southern California has jumped from 12,000 to 220,000. This development seriously encroached on the habitat of the Coachella Valley fringe-toed lizard and the free-flowing sand that sustained it. In 1980, the U.S. Department of the Interior listed the lizard as "threatened" under the Endangered Species Act and construction came to a virtual standstill. Since then developers, environmental organizations, and local, state, and federal agencies have worked together to establish and maintain a self-sustaining ecosystem (the Coachella Valley Preserve); the fringe-toed lizard is an integral part of this ecosystem. Developers pay a fee as compensation for the development of private land outside the preserve. The fees collected are then used to acquire lands for the preserve. The Nature Conservancy manages the day-to-day operations, guided by a committee of agencies and groups. A federal ranger helps control unauthorized vehicle use and other violations.The Santa Rosa Mountains, designated as a National Scenic Area in 1990, are another example of public and private collaboration to protect sensitive resources. The area is home for such endangered species as the desert slender salamander and peninsular bighorn sheep, as well as more than 500 species of plants. It has provided sustenance for Indians for thousands of years.

Southern California's Santa Rosa Mountains are a National Scenic Area and the home to many endangered species. They are also a sacred area to Indians. When water flows through the desert it provides a unique habitat. The San Pedro River, a perennial stream, sustains one of the richest wildlife populations in the United States. The summer tanager, a visitor to the San Pedro riparian habitat, is a neotropical migratory bird with a robinlike song. Its diet consists of insects and fruits.

Photo by BLM

The area encompasses approximately 80,000 hectares of the Santa Rosa and San Jacinto Mountains. The federal government, the state of California, and Indian tribes own approximately 70 percent of this area. Because the remaining privately owned land includes important areas critical to the protection of wildlife and other resources, government agencies, conservation organizations, and other entities have been working since 1990 to acquire private lands in the Santa Rosas and involve the public in the preparation of a long-term resource management plan for this area. In April 1996, a visitor center opened with the help of the city of Palm Desert and local volunteers who help staff the center. Large-scale, long-term cooperative efforts such as the Coachella Valley Preserve and the Santa Rosa National Scenic Area will become more and more important as the population surrounding the Sonoran Desert increases and demands for uses of the desert continue to grow.

American Indian Traditional Cultural PlacesMany American Indian traditional cultural places are located in the Sonoran Desert, within tribal reservations but also on the public lands. Information about such areas is often very sensitive and, by tradition, is not shared with uninitiated members of a tribe or with people outside the tribe. This makes it difficult for managers to protect these places when making decisions about actions that could damage or destroy them.Even when sacred areas are known, issues can arise regarding their designations and uses. Places valued by Indians for cultural reasons are often valued by other groups, such as recreationists or commodity interests, for different reasons. In these situations, land managers strive for compromise, but sometimes this is not easily achieved.

Riparian AreasRiparian areas - the lush green areas of vegetation bordering lakes, ponds, rivers, streams, and wet meadows - are lifelines in the desert. More than 90 percent of the historic riparian areas in the Southwest have been lost, in large part as a result of the damming of rivers.One of the most important riparian areas in the United States runs through the Chihuahuan Desert and the Sonoran Desert in southeastern Arizona along the San Pedro River. Despite the dryness of the surrounding land, it sustains one of the richest wildlife populations in the country. To protect its remarkable resources, federal land managers acquired a 58-kilometer segment of the San Pedro River north of the Arizona-Mexico border in 1986.

When water flows through the desert it provides a unique habitat. The San Pedro River, a perennial stream, sustains one of the richest wildlife populations in the United States

Photo by Diane Drobka

Legislation passed by Congress in 1988 designated this segment of more than 22,400 hectares a Riparian National Conservation Area. This "ribbon of life" represents the most extensive, healthy riparian ecosystem remaining in the desert Southwest.The conservation area is home to 82 species of mammals, 12 species of fish, 47 species of reptiles and amphibians, and 100 species of breeding birds. It provides an invaluable habitat for another 250 species of migrant and wintering birds.Some of these birds, called neotropical migratory birds, breed in North America and winter south of the United States. Birds inhabiting the area include yellow-billed cuckoos, Crissal thrashers, green kingfishers, vermilion flycatchers, elf owls, Lucy's warblers, Botteri's sparrows, yellow-breasted chats, Bell's vireo, and gray hawks. The gray hawks on the San Pedro River represent about 30 percent of the nesting gray hawks in the United States.

The summer tanager, a visitor to the San Pedro riparian habitat, is a neotropical migratory bird with a robinlike song. its diet consists of insects and fruits.Photo by BLM

The same water and vegetation that attracts animals to the San Pedro River has attracted humans for thousands of years. In fact, the area contains archaeological sites representing the remains of human occupation from before the end of the Pleistocene, 11,200 years ago, up to the present. The earliest sites contain the remains of mammoths and bison with the stone tools of the people who hunted them. Archaeologists call these earliest big game hunters the Clovis people. Of the handful of Clovis sites in North America, six have been found in this area.After the Clovis people, other Native Americans lived along the San Pedro River, eventually encountering the first Europeans to enter the area. Francisco Vasquez de Coronado likely followed the San Pedro River in 1540 on his exploratory expedition into what is now Arizona. More than 200 years later, Spain built a fort, or Presidio, beside the San Pedro River to protect the northern border of its New World empire. Remnants of the Presidio, Santa Cruz de Terrenate, still stand (See "Science in Process: Discovering the Past at Santa Cruz,"Science and Children, April 1992 for more information about the Presidio.) Miners later built millsites. In the 1880s, a railroad was constructed along the river, and the Fairbank townsite was built to serve as a depot. Now a "ghost town" that is slowly being restored, Fairbank is a point of interest for the recent influx of tourists who ride the train today.Some of the activities that affect the riparian ecosystem include water pumping for agricultural and residential purposes, livestock grazing, sand and gravel operations, firewood cutting, vehicle traffic, and recreational activities. If not properly managed, these activities could adversely affect the water quality, riparian vegetation, wildlife, archaeological sites, fossils, and soils.After working with the public to consider various alternatives, land managers have adopted a series of actions to protect this important riparian ecosystem. Since the primary purpose for obtaining the lands along the San Pedro River was to enhance the riparian ecosystem, land managers have prohibited livestock grazing for at least 15 years. Sand and gravel extraction will also be prohibited in the riparian zone. Motor vehicles and mountain bikes are restricted to designated roads, and campground stays are limited to seven days. To improve wildlife habitat, managers will develop ponds and marshes, plant trees and other native vegetation, install nest boxes, and work with others along the San Pedro on water usage and conservation.Interpretive displays and trails as well as environmental education field study areas will inform the public about these sensitive resources. Three areas designated as "Areas of Critical Environmental Concern" will be managed with special care. Developed campgrounds, picnic sites, and historic and archaeological sites for the public to visit will accommodate the growing demands for recreation. With the adoption of these and many other protective measures, land managers hope to preserve the integrity of this critical riparian ecosystem long into the future.The benefits of protecting the health of the San Pedro riparian ecosystem and other riparian areas throughout the West are extensive. When a riparian area is healthy and functioning, its lush vegetation slows the flow of water, allowing it to soak into the soil and recharge the water table. In many cases, as groundwater storage is enhanced, intermittent streams begin to flow year-round. Riparian vegetation along upper watershed streams can absorb and dissipate the energy of floodwaters, reducing downstream damage. Plant roots bind and hold settled soils in place while stems and foliage filter and settle sediments, resulting in the building and protection of stream banks. Healthy riparian areas also reduce sedimentation in reservoirs, extending their effective life and improving water quality. They also help control nonpoint-source pollution.The restoration of perennial streamflow provides water for wildlife during the normally dry summer months, reducing the need for expensive water developments.Many wildlife species depend on the unique and diverse habitats offered by riparian areas. This habitat provides food, water, shade, and cover - necessities for wildlife and livestock. Riparian vegetation is of critical importance to fish. It provides escape cover, lowers summer water temperatures through shading, and reduces stream bank erosion that can lead to silt in spawning and rearing areas. Enhancing riparian areas also increases opportunities for recreation, such as camping, hiking, bird watching, photography, fishing, hunting, and simple relaxation.

Looking to the FutureThe Sonoran Desert is a magnificent but fragile ecosystem. Many of the threats facing this environment relate to how people choose to use the resources of this land. If treated with respect and care, it will be a place of beauty and diversity for many generations. Educating today's youth about the area's many distinctive plants, animals, and other natural features will help them make good decisions about managing the resources of the desert in the future.

The minerals of the Sonoran Desert are very important to the economy of Arizona as well as the United States. In 1995, the value of nonfuel production in Arizona, almost entirely from the Sonoran Desert, was about $3 billion, the highest in the nation. Production of copper from copper-bearing minerals of Arizona's Sonoran Desert accounts for about 60 percent of the nation's copper output. The Sonoran Desert has examples of minerals-such as wulfenite (beautiful reddish-orange crystals), the gemstone peridot, malachite, and azurite-which are prized by both amateur and professional collectors.The blue color of this sample of malachite and azurite is characteristic of copper-bearing minerals. This specimen is from the Czar Shaft Mine in Bisbee, Arizona. Photo by J .Scovil